1. Field of the Invention
The present invention relates to a visual sensor for observing all directions, and more particularly, it relates to an omnidirectional visual sensor utilizing a mirror with a surface of revolution.
2. Description of the Background Art
Conventionally known omnidirectional visual sensor utilizing a mirror with a surface of revolution include a visual sensor utilizing a conical mirror, a spherical mirror, a hyperboloidal mirror or a paraboloidal mirror. Referring to FIG. 1, an omnidirectional visual sensor employing a hyperboloidal mirror includes a vertically placed hyperboloidal mirror facing downward and a camera vertically placed under the mirror facing upward. The hyperboloidal mirror (hyperboloid of two sheets) has a geometric property of having two focal points, and a certain geometrical relation holds between the two focal points and the hyperboloid thereof. When aligning the lens center of the camera with the first focal position of the hyperboloid, therefore, it is possible to form an optical system such that extension of light from an arbitrary point P in the environment necessarily passes through the second focal point B of the mirror regardless of the position thereof as shown in FIG. 1, i.e., a perspective transformation coordinate system. In other words, when setting the principal point of the photoreceiving lens system on the second focal point B of the hyperboloidal mirror, an image acquired in the omnidirectional visual sensor employing the hyperboloidal mirror can be readily transformed into a coordinate system having the first focal point C of the mirror as a temporary lens center. An omnidirectional visual sensor utilizing a conical or spherical mirror cannot perform such transformation.
Details related to a conventional omnidirectional visual sensor employing a hyperboloidal mirror are described in Yamazawa et al., "Omnidirectional Imaging with Hyperboloidal Projection", Proceedings of the 1993 IEEE/RSJ International Conference on Intelligent Robots and Systems, Yokohama, Japan, Jul. 26-30, 1993, and the description thereof is incorporated herein by reference.
An omnidirectional visual sensor utilizing a paraboloidal mirror exhibits a property similar to that of the hyperboloidal mirror under a certain condition. When a photoreceiving system allowing a parallel projection can be prepared for a convex paraboloidal mirror, the viewpoint is located on a single point similarly to the hyperboloid for enabling expression in a spherical coordinate system having the origin on this point. In the conventional study, however, a correct perspective transformation coordinate system cannot be implemented due to employment of a photoreceiving system such as an optical system having a long focal length or a telecentric lens approximately allowing a parallel projection.
Details related to a conventional omnidirectional visual sensor utilizing a paraboloidal mirror are described in U.S. Pat. No. 5,760,826, and the description thereof is incorporated herein by reference.
On the other hand, every mirror with a surface of revolution generally has a problem of blurring due to influence by astigmatism or the like, thus requiring close attention in design of the optical system. In general, it is difficult to reduce the influence by the astigmatism with a single mirror or lens, and a plurality of lenses must be combined with each other. In general, there has been proposed an optical system like a reflecting telescope utilizing two mirrors having optimum curvatures employing a method of acquiring a virtual image formed by a secondary mirror with a general telecamera through a primary mirror in the form of a two-dimensional torus in order to minimize the influence by the astigmatism.
The details thereof are described in Takeya et al., "Reflecting Wide-Angle Optical System", Proceedings of Annual Conference sponsored by the Society of Instrument and Control Engineers (SICE), Tokyo, Japan, Jul. 26-28, 1994, and the description thereof is incorporated herein by reference.
However, the aforementioned omnidirectional visual sensor employing a hyperboloidal mirror has a problem of blurring due to influence by astigmatism or the like. Therefore, it is difficult to implement a small visual sensor which can transform a central projection while obtaining a sharp image by merely adjusting the shape of the hyperboloidal mirror and the optical system of the telecamera forming the photoreceiving system.
Further, it is hard to make the omnidirectional visual sensor utilizing a paraboloidal mirror smaller similarly to the omnidirectional visual sensor employing a hyperboloidal mirror, and it cannot correctly transform a central projection due to an approximate parallel projection by an optical system having a long focal length or a telecentric lens.
In addition, the aforementioned reflection wide-angle optical system cannot transform a central projection dissimilarly to the omnidirectional visual sensor employing a hyperboloidal mirror, although it can optimize the imaging system.